1. Field of the Invention
The present invention relates to a wide band-width high NA optical fiber which is used, for example, as an optical transmission line in an intermediate distance high speed optical data link or a computer link.
2. Description of the Related Art
Hitherto, as a technique for enlarging a band-width of a multimode optical fiber, there is known a method for decreasing a mode dispersion by forming a refractive index profile of a core of the optical fiber in the form of a square distribution. A graded index (GI) type quartz core/quartz cladding optical fiber in which an additive such as Ge is used as a dopant to achieve the square distribution type refractive index profile of the core is widely used (see, for example, Gloge and Marcatili, "Multimode Theory of Graded-Core Fibers", The Bell System Technical Journal, 52 (9), 1563-1578 (1973)).
When the GI type quartz optical fiber is designed so as to increase NA (numerical aperture) which is an important structural parameter of the optical fiber for improving an optical coupling efficiency with a light source for optical communication and a bending loss characteristics, a concentration of the dopant such as Ge in the core quartz glass should be increased. Then, in addition to increase of a production cost of the optical fiber due to the use of a rare element such as Ge, a production yield of the optical fiber during drawing of the optical fiber is decreased. Further, it is very difficult to precisely control the dopant concentration to achieve the square distribution of the refractive index profile in comparison with an optical fiber having a low NA. In addition, disturbance of the refractive index profile increases the mode dispersion of transmitted signals so that a band-width of the optical fiber is narrowed.
In contrast, as one measure for designing a high NA quartz optical fiber at a low cost, there is a so-called truncated type optical fiber which has a refractive index gap between a peripheral part of the core and the cladding. In this optical fiber, there is a trade-off relationship between the NA characteristic and the band-width characteristic, and the NA is not increased while the band-width is decreased.
As a technique for providing a high NA optical fiber with a large core diameter and high strength at a low cost, a plastic cladding fiber (PCF) comprising a core made of a glass material and a plastic cladding has been proposed for a long time, and a step index type optical fiber in which a core refractive index is uniform is practically used.
On the increase of band-width by utilizing a so-called mode filter effect of the PCF, some studies have been made (see, for example, Nakahara et al, "Transmission Loss Characteristics of Plastic Cladding Fiber", Optics Quantum Electronics Study Group, Document No. OQE 73-91 (1974) 134). However, the proposed optical fiber increased the band-width with sacrifice of the transmission loss and the NA of the fiber.
Hitherto, design factors which can increase both the NA and the band-width have not been found.